1. Field of the Invention
The present invention relates to a zoom lens system to be used in a small and light-weight video camera or digital camera and the like having the f-number (f/D: f: the focal length; D: a diameter) of more than 1:2.8 at the short focal length extremity, and a zoom ratio of 2.5 to 4.0, and in particular, relates to a zoom lens system which can make the overall length thereof, at the retracted position, shorter.
2. Description of the Prior Art
In recent years, along with further miniaturization and integration of imaging devices and the elements contained therein, the zoom lens system used in a video camera and an electronic still camera have been required to be further miniaturized and to have higher quality.
Furthermore, a solid-state imaging device, such as a CCD, which is used in a video camera or electronic still camera and the like is provided with a color separation filter in the close vicinity of a light receiving surface thereof. Accordingly, if a bundle of light rays emitted from the zoom lens system is inclined with respect to the optical axis, the bundle of light rays is blocked by the color separation filter, i.e., an eclipse occurs, so that the amount of peripheral illumination is reduced and unevenness of color occurs due to the characteristics of the color separation filter. Therefore in order to achieve suitable telecentricity, namely, in order to make a bundle of light rays incident on the light receiving surface at a right angle, an optical system in which the exit pupil is positioned distant from the image plane is required.
Still further, in a recent zoom compact camera, it is important to make the size thereof smaller when the photographing lens system is advanced from the retracted position. In addition, compactness of the camera when the photographing lens system is retracted, i.e., in the retracted position, has also become important so that a user can easily carry the camera with him/her. In other words, it is desirable for the above-explained zoom lens system to be thinner when retracted. For this purpose, and in order to reduce mechanical load on the zoom lens system, it is required to make the traveling distances of the lens groups thereof much shorter.
In a compact zoom lens system of the prior art, a two-lens-group zoom lens system, which includes a negative first lens group and a positive second lens group, is known. However, such a two-lens-group zoom lens system is not desirable for a zoom lens system in which a solid-state imaging device, such as a CCD, is used, since the exit pupil is close to the image plane.
As an attempt to overcome the above drawbacks, a three-lens-group zoom lens system in which a positive lens group, which is either immovable or movable, is provided between the second lens group and the solid-state imaging device has been proposed, as disclosed in Japanese Unexamined Laid Open Patent Publications (hereinafter, JP) No. Hei-6-94996, No. Hei-10-39214 and No. Hei-11-194274.
The zoom lens system of JP-6-94996 has disclosed improved telecentricity due to the three-lens-group arrangement; however, the zoom ratio is still about 2.0, and requirements of a high zoom ratio is not yet satisfied.
The lens arrangements of JP-10-39214 and JP-6-94996 are the same, and both satisfy the zoom ratio of 3. However, compactness when the zoom lens system is retracted is not yet satisfactorily attained, since the overall length of the zoom lens system is longer with respect to the focal length.
In JP-11-194274, the zoom lens system includes the first lens group constituted by the two lens elements with at least one aspherical surface formed thereon, and attains the zoom ratio of 3 with the seven lens elements constituting the three lens groups. In other words, miniaturization of the zoom lens system has been attained. However, relatively large spherical aberration and curvature of field occur, with respect to the focal length. Therefore the zoom lens system of JP-11-194274 cannot satisfy requirements of higher resolution in proportion to a finer pixel size in recent years. Moreover, since the second lens group is constituted by four lens elements, there still is room for improvement on cost reduction.
The present invention provides a zoom lens system which (i) is used in a small and light-weight video camera or digital camera and the like having the f-number of more than 1:2.8 at the short focal length extremity, and a zoom ratio of about 2.5 to 4.0, (ii) has the half angle-of-view of more than 30xc2x0 at the short focal length extremity, (iii) has sufficient image-forming performance suitable for a solid-state imaging device of higher resolution, and (iv) can be made much compact when retracted.
As an aspect of the present invention, there is provided a zoom lens system including a negative first lens group, a positive second lens group, and a positive third lens group, in this order from the object. Zooming is performed by moving the first to third lens groups respectively along the optical axis. The zoom lens system satisfies the following conditions:
1.3 less than |f1/f2| less than 1.8 xe2x80x83xe2x80x83(1) 
2.0 less than f3/fw less than 3.4 xe2x80x83xe2x80x83(2) 
1.05 less than m3t/m3w less than 1.25 xe2x80x83xe2x80x83(3) 
wherein
fw designates the focal length of the entire the zoom lens system at the short focal length extremity;
f1 designates the focal length of the negative first lens group;
f2 designates the focal length of the positive second lens group;
f3 designates the focal length of the positive third lens group;
m3w designates the magnification of the positive third lens group at the short focal length extremity, when an object at infinity is in an in-focus state;
m3t designates the magnification of the positive third lens group at the long focal length extremity, when an object at infinity is in an in-focus state.
In the positive second lens group, the most image-side surface thereof is a divergent surface constituted by a concave surface, and satisfies the following condition:
0.4 less than R/fw less than 1.0 xe2x80x83xe2x80x83(4) 
wherein
R designates the radius of curvature of the most image-side surface of the positive second lens group.
For example, the positive second lens group includes a positive lens element with at least one aspherical surface, and cemented lens elements having a positive lens element and a negative lens element, in this order from the object. In other words, the two lens groups are constituted by the three lens elements. The aspherical surface is formed so that the positive power becomes weaker toward the periphery than the positive power generated by a spherical lens surface having a radius of curvature in the paraxial region. The positive second lens group satisfies the following conditions:
xe2x88x920.1 less than Npxe2x88x92Nn less than 0.2 xe2x80x83xe2x80x83(5) 
5 less than vpxe2x88x92vn less than 30 xe2x80x83xe2x80x83(6) 
wherein
Np designates the refractive index of the d-line of the positive lens element of the cemented lens elements;
Nn designates the refractive index of the d-line of the negative lens element of the cemented lens elements;
vp designates the Abbe number of the positive lens element of the cemented lens elements; and
vn designates the Abbe number of the negative lens element of the cemented lens elements.
On the other hand, the negative first lens group can be constituted by a negative meniscus lens element having the convex surface facing toward the object, a negative lens element, and a positive lens element having a convex surface facing toward the object, in this order from the object. In this case where the three lens groups are constituted by the three lens elements, each lens surface of the first lens group can be formed as a spherical surface. Alternatively, the negative first lens group can be constituted by a negative lens element having a concave surface facing toward the image, and a positive lens element having a convex surface facing toward the object, in this order from the object. In other words, the two lens groups are constituted by the two lens elements. Furthermore, in the negative first lens group, at least one aspherical surface, on which the positive power becomes stronger toward the periphery than the positive power generated by a spherical lens surface having a radius of curvature in the paraxial region, can be provided.
Focusing can be carried out by either the negative first lens group or the positive third lens group. Note that constituting the positive third lens group by a biconvex single lens element is cost-effective.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-78881 (filed on Mar. 19, 2001) which is expressly incorporated herein in its entirety.